This doesn't happen everytime I print, but sometimes the Colour Scheme legend shows a purple hatched box at the bottom and the text "Calculating" next to it.

I found the issue referenced in this support post.
It was supposed to be fixed in Revit 2019.1 but I'm still experiencing the issue with Revit 2020.2.8
The initial workaround was to open each sheet before printing and wait for the calculations to be done. This is alright if you have a small number of sheets, it's also hard to remember when you're printing in a hurry.
Does anyone know if there is an update I'm missing that solves this?